The present invention relates to a method for manufacturing composite membranes, the composite membranes manufactured according to this method, composite membranes containing at least one siliceous reinforcing material and at least one organic proton conductor, and to the use of these composite membranes in PEM fuel cells.
Membranes for PEM fuel cells must have sufficient chemical and mechanical stability, high proton conductivity, and they must be inexpensive to manufacture.
The perfluorinated cation-exchange membranes used till now show considerable deficiencies in this regard. Besides the complex manufacturing method and the recycling problem, this material is very expensive and has a high methanol permeability, which markedly limits the use of these membranes, for example, in direct methanol fuel cells (DMFC).
Further membrane materials are modified, high-temperature resistant polymers, such as polybenzimidazole (PBI) and polyethersulfone (PES). To this end, the PBI is usually treated with phosphoric acid. The phosphoric acid molecules are, on one hand, attached to the polymer by hydrogen bridge bonds and, on the other hand, bound in the membrane to protonate the imidazole group. However, it is problematic that the phosphoric acid is gradually separated from the PBI matrix with the water produced during the operation of the fuel cell. Moreover, the PBI phosphoric acid membrane has a very low E-module, which is why unsatisfactory stability of the membrane is to be expected in fuel cells.
Inexpensive alternative materials based on sulfonated aryl polymers as, for example, sulfonated PEEK, PEK and PES, are known from European Patent Application EP 0 574 791 A1. However, the cation-exchange membranes made from such sulfonated aryl polymers exhibit strong swelling properties at elevated temperature. Therefore, the suitability of such membranes for use in a fuel cell system is strongly limited.
German Patent Application 44 22 158 A1 describes composite membranes made of sulfonated polyetherketone (PEK) and unmodified polyethersulfone (PES). The two components are completely miscible with one another, which is attributable to their very similar chemical structures and to the polarity of PES (ion-dipole interactions). However, this interaction arising from similarity of structure appears to be still insufficient so that there is a risk for these membranes to swell strongly at elevated temperature when operating with an ion-exchange capacity as is required in the operation of fuel cells. Described are three or four component mixtures of sulfonated PEK, PES, polyvinyl pyrrolidone (PVP) und polyglycol dimethyl ether (PG) that have a better water absorption. However, no quantitative information is provided on water absorption.
German Patent Application DE 198 17 374 A1 describes mixtures of sulfonated aryl polymer (PEEK and PSU) and polybenzimidazole (PBI) which are covalently cross-linked due to the proton transfer from the sulfonated aryl polymer to the PBI (e.g., PEEK-SO2—O—H—N-PBI). This cross-linking occurs already at room temperature in the solvent, for example, N-methyl pyrrolidone (NMP), forming an insoluble polyelectrolyte complex. In order to manufacture composite membranes, the sulfonated aryl polymer must be converted into a soluble salt form. This additional step complicates the manufacture of the membrane.
The interaction between PBI and aryl polymer is so strong that high inhomogeneities can be caused in the membrane between the cross-linking region, the water-swollen gel phase, and the polymer matrix. Consequently, internal stresses can develop in the membrane, which can deteriorate the mechanical stability of the membrane.
Also known in the prior art are composite membranes made of sulfonated aryl polymer (PEEK or PSU) with aminated polysulfone (PSU). Since aminated polysulfones are weak polybases, it is possible to produce polyacid-base mixtures in the solution. Both ionic interactions and hydrogen bridge bonds act between the composite components, i.e. ring structures that are physically crosslinked. These composite membranes were tested in PEM fuel cells and direct methanol fuel cells. In the process, current densities of 1.0 to 1.2 A/cm2 developed in H2/O2 PEM fuel cells at a voltage of 0.7 V, and of 0.4 to 0.6 A/cm2 in air/H2 PEM fuel cells. In direct methanol fuel cells, this membrane showed an i-U curve comparable to that of, for example, Nafion 117.